Such a circuit arrangement is disclosed in DE 103 33 820.9 from the same applicant as the present application. The disclosure content of this application, particularly with regard to the prior art described there as well as the statements on phase modulation, is incorporated into the present application by this reference. However, the circuit arrangement presented there for carrying out a phase modulation is attended by very high outlay on implementation that is reflected in the costs.
As is therefore known to the person skilled in the art, the frequency of the base signal is selected such that said frequency is between the first azimuthal resonant mode and the first radial acoustic resonant mode of the lamp. Moreover, the third frequency is selected such that the latter corresponds approximately to the second longitudinal resonant frequency of the lamp.
In relation to the prior art, reference may further be made to U.S. 2003/0102824 A1, which discloses a circuit arrangement for the high-frequency operation—the carrier frequency being between 400 and 600 kHz—of a high pressure discharge lamp. As is known to the person skilled in the art, during operation of a high pressure discharge lamp with a carrier frequency in this frequency range there is no need to take any measure for arc straightening. In DE 103 33 820.9, the sweeping of the carrier frequency serves the purpose of arc straightening. In order to prevent color segregation, in the abovementioned U.S. 2003/0102824 A1 a low-frequency signal of the order of magnitude between 20 and 30 kHz and which corresponds to the second longitudinal resonant frequency is used to switch to and fro between two carrier frequencies. The frequency range used in this publication for the carrier frequency is, however, not desired in practice since it is attended by emission problems. It would not be possible to implement an operation of the circuit arrangement proposed in the abovementioned US document with carrier frequencies in the range around 50 kHz, since the required filter characteristic would be associated with a high outlay on components given such a low frequency. The modulation frequency is coupled to the carrier frequency in a fixed fashion owing to the filter used and to the sweeping.